Negative North Atlantic Oscillation Index Research Articles

Seasonal and interannual variability of the surface circulation in the eastern Gulf of Cadiz (SW Iberia)

An 11‐year (1996–2007) time series of current meter observations representative of the open sea circulation; a 4‐year (2001–2005) time series of current meter records over the continental shelf and in situ data during different seasons have been compared in order to study the seasonal and interannual variability of the surface circulation in the eastern Gulf of Cadiz. The open sea velocity observations indicate southeastward flow along the northern continental slope of the Gulf of Cadiz, compatible with anticyclonic circulation, during most of the year and more intense during summer months. Flow reversals (northwestward circulation) at seasonal time scales in late autumn and early winter (preferably December and January) are a rather recurrent feature with variable intensity depending on the year. Anticyclonic circulation is associated with westerlies, whereas flow reversals usually take place under easterly episodes, suggesting wind‐driven circulation. Negative North Atlantic Oscillation indices (indicative of southward displacement of the Azores high) are also linked to the reversals. Changes in this mainly wind‐driven large‐scale surface circulation are echoed by the shelf circulation: the coastal countercurrent that closes the mesoscale cyclonic cell over the eastern shelf in spring‐summer (upwelling season) is replaced by and eastward current in autumn and winter.

The Little Ice Age glacier maximum in Iceland and the North Atlantic Oscillation: evidence from Lambatungnajökull, southeast Iceland

This article examines the link between late Holocene fluctuations of Lambatungnajokull, an outlet glacier of the Vatnajokull ice cap in Iceland, and variations in climate. Geomorphological evidence is used to reconstruct the pattern of glacier fluctuations, while lichenometry and tephrostratigraphy are used to date glacial landforms deposited over the past /400 years.Moraines dated using two different lichenometric techniques indicate that the most extensive period of glacier expansion occurred shortly before c. AD 1795, probably during the 1780s. Recession over the last 200 years was punctuated by re-advances in the 1810s, 1850s, 1870s, 1890s and c. 1920, 1930 and 1965. Lambatungnajokull receded more rapidly in the 1930s and 1940s than at any other time during the last 200 years. The rate and style of glacier retreat since 1930 compare well with other similar-sized, non-surging, glaciers in southeast Iceland, suggesting that the terminus fluctuations are climatically driven. Furthermore, the pattern of glacier fluctuations over the 20th century broadly reflects the temperature oscillations recorded at nearby meteorological stations. Much of the climatic variation experienced in southern Iceland, and the glacier fluctuations that result, can be explained by secular changes in the North Atlantic Oscillation (NAO). Advances of Lambatungnajokull generally occur during prolonged periods of negative NAO index. The main implication of this work relates to the exact timing of the Little Ice Age in the Northeast Atlantic. Mounting evidence now suggests that the period between AD 1750 and 1800, rather than the late 19th century, represented the culmination of the Little Ice Age in Iceland.

Inter-decadal variability in zooplankton and phytoplankton abundance on the Newfoundland and Scotian shelves

Abstract Continuous Plankton Recorder (CPR) sampling on the Newfoundland and Scotian shelves covers three multi-year periods characterised by negative (1962–1971), positive (1992–2000) and negative/neutral (2001–2003) values of the NAO index. Water temperatures respond differently to changes in the NAO in different regions: a positive NAO index tends to lead to reduced temperatures on the Newfoundland shelf and to increased temperatures on the central/western Scotian shelf, and a negative NAO index to the reverse. Since the 1960s, the hydrographic changes due to the NAO have been superimposed on a freshening of the water column throughout the region, which is attributed to increased contribution of Arctic water outflow. Changes in plankton abundance measured by the CPR for the three time periods were generally, but not always, similar on the Newfoundland and eastern and western regions of the Scotian shelf, although Arctic species (e.g. Calanus glacialis , Calanus hyperboreus ) were notably more abundant and warm water species (e.g. Metridia lucens , euphausiids) less abundant on the Newfoundland shelf than on the Scotian shelf. Three categories of phytoplankton (colour, diatoms, dinoflagellates) increased in abundance in the 1990s, and these increases generally persisted into 2001–2003. This is believed to be a response to the persistent freshening of the water column, probably due to increased stratification. The Arctic species C. glacialis and C. hyperboreus also showed persistent increases in abundance after 1992, perhaps due to increased transport from the Arctic, although the abundance of the Arctic slope water species Metridia longa decreased. Two groups, Calanus 1–4 and euphausiids, both thought to play important roles in the food chain, showed persistent decreases in abundance after 1992, especially on the Newfoundland shelf. In all regions, Calanus finmarchicus 5–6, Oithona spp. and Centropages hamatus abundance changed in association with variations in the NAO, although no common mechanism could be identified. C. finmarchicus 5–6 abundance decreased in the 1990s and increased after 2001, while the other two species showed the opposite pattern. Centropages typicus and M. lucens abundance on the Scotian shelf increased with rising temperature. This is attributed to increased production rates for the former and an increased influx of warm, M. lucens- rich, slope water on to the shelf for the latter. A comparison between ring net and CPR sampling on the Newfoundland shelf suggests that the Calanus 1–4 category is dominated by C. finmarchicus and that late stage C. glacialis and C. hyperboreus are grossly under-sampled compared to late stage C. finmarchicus .

Winter Lightning and North Atlantic Oscillation

Abstract The relationship between the North Atlantic Oscillation (NAO) and total (in cloud and cloud to ground) lightning activity during the wintertime (December to March) is analyzed. Lightning data from the Optical Transient Detector were used and were studied in the North Atlantic and western European area. The southern half of the area analyzed (latitudes lower than ∼50°N) was observed to have an increased (decreased) lightning flash rate associated with negative (positive) NAO index values. The opposite relationship was found when the northern half (latitudes higher than ∼50°N) was considered. In both cases, the effect of the NAO on the lightning activity was stronger over the eastern area analyzed. Low (clearly <1) absolute values of the NAO index seem to be associated with minimum lightning activity.

The Outstanding 2004/05 Drought in the Iberian Peninsula: Associated Atmospheric Circulation

Abstract The 2004/05 hydrological year (October 2004 to September 2005) was characterized by intense dry conditions affecting most of western Europe (35°–55°N and 10°W–10°E). In Iberia the drought affected every month of this period, with the southern half of Iberia receiving roughly 40% of the usual precipitation by June 2005. Moreover, this episode stands as the driest event in the last 140 yr, producing major socioeconomic impacts particularly due to the large decrease in hydroelectricity and agricultural production in both Iberian countries (Portugal and Spain). To assess the atmospheric submonthly circulation associated with this drought an Eulerian [weather types (WTs)] and a Lagrangean (objective storm tracks) analysis were combined. There was a dramatic drop in “wet” WT frequency during winter, with less than 50% of the normal value, and a corresponding increase of “dry” WTs. The storm-track analysis reveals an impressive northward displacement of cyclone trajectories in the North Atlantic sector in winter months, resulting in an almost complete absence of cyclones crossing Iberia and western Europe. At the monthly scale, the intense drought in Iberia was due to a combination of different physical mechanisms. First, the scarce precipitation observed between November 2004 and January 2005 was associated with positive North Atlantic Oscillation (NAO) indexes for these months. In February, the East Atlantic (EA) pattern seems to be the main driver. In March neither the negative NAO (−1.8) nor the positive EA (1.1) are capable of explaining the large negative precipitation anomalies. However, it is shown that during March 2005, an intense and anomalous blocking was displaced southward of its usual location, inhibiting the occurrence of precipitation over Iberia and leading to a negative NAO index anomalously associated with low precipitation records.

Changing climate and extreme floods in the British uplands

Recent devastating flood events in the British uplands have focused attention on flood prediction and risk assessment, and highlighted one of the most important challenges facing river basin managers in the UK. The short instrumental record of river flow in the UK and the sparse distribution of gauges in upland river basins hamper predictions of future flooding. In this study, we compare and evaluate all lichen‐dated flood deposits from UK upland river basins in order to assess changing flood risk over the last 250 years. Our analysis indicates that the incidence and size of extreme events have markedly decreased over the last 50 years, and may be at their lowest level since ad 1750. Occurrence of extreme upland floods over the last 200–300 years appears to be associated with negative North Atlantic Oscillation index values and significant non‐stationarity evident in the historical flood series is shown to relate to decadal and multi‐decadal scale climatic fluctuations.

Negative NAO and cold Eurasian winters: how exceptional was the winter of 1962/1963?

Since the mid 1980s frequent positive phases of the North Atlantic Oscillation (NAO) linked to a higher-than-normal pressure difference between the Azores high and Icelandic low pressure centres and stronger westerlies (Hurrell, 1995; Thompson and Wallace, 1998; Osborn, 2006) have led to higher winter temperatures over most of western Europe. Harsher winters were more frequent between the 1950s and the 1980s (see Figure 9 in Graham et al., 2006). Even though mild winters similar to the recent ones did occur they alternated with colder winter seasons. The best remembered is probably the winter of 1962/1963 when temperatures were well below zero across most of Europe. Figure 1 illustrates the unusual amounts of snow that occurred at lower elevations in Switzerland. With the exception of western Ireland, most of the British Isles was covered by a blanket of snow for most of the winter (Met Office, 2006a). On an anecdotal note, the harsh conditions prevailing during that winter led a then relatively unknown Bob Dylan, who was staying in a London flat at that time, to burn furniture in order to keep warm (Williamson, 2004). On mainland Europe it was the only time in the twentieth century that large lakes such as Lake Constance or Lake Zurich were completely ice-covered and it was only in March that the ice finally broke up (Pfister, 1999). Cold winters such as the one in 1962/1963 are generally associated with a negative NAO index characterized by weak westerlies (lower-than-normal pressure difference between Icelandic low and Azores high) and a reduced Atlantic influence in western Europe. The NAO index, and the late spring seasurface temperature pattern in the North Atlantic, can be used as indicators to forecast the conditions during the following winter (Graham et al., 2006; Met Office, 2006b). However, prediction of how much colder or warmer the winter season is going to be and in what regions the anomalies are likely to be most pronounced is subject to large uncertainties. A negative NAO index during the winter months does not always coincide with very cold winter conditions over the same regions. A recent example is the winter of 2005/2006. Despite the clearly negative NAO index the winter temperatures over most of Britain remained close to average values and were clearly higher than those experienced in 1962/1963. However, the winter 2005/2006 was the coldest winter since 1986 in parts of central, eastern and southern Europe. This illustrates that even if the NAO index gives an indication of whether a winter is warmer or colder than normal, the spatial pattern of the largest temperature anomalies can vary. In this study, we use reanalysis data from the National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR, Kalnay, et al., 1996) for surface temperature and sealevel pressure to compare the winter of 1962/1963 with other winters associated with a negative NAO index and very low temperatures over other regions of Eurasia. The purpose is to highlight how exceptional the winter of 1962/1963 was compared to other harsh winters in terms of amplitude and spatial extent of the cold temperature anomaly. The fact that the winter of 1962/1963 is so well remembered is partly due to the character of an average winter over most areas of Britain and western Europe; generally temperatures are above freezing and long spells of snow are rare. However, a shift in temperature by a few degC can transform the traditional green British winter into a snowy (white) one, thus making the difference even more obvious. In comparison, other areas of Europe/ Eurasia are characterised by average winters where snow and ice are common. Therefore, negative temperature anomalies are more likely to go unnoticed; a winter may be colder than normal but it does not change its colour.

The 1986–1989 ENSO cycle in a chemical climate model

Abstract. A pronounced ENSO cycle occurred from 1986 to 1989, accompanied by distinct dynamical and chemical anomalies in the global troposphere and stratosphere. Reproducing these effects with current climate models not only provides a model test but also contributes to our still limited understanding of ENSO's effect on stratosphere-troposphere coupling. We performed several sets of ensemble simulations with a chemical climate model (SOCOL) forced with global sea surface temperatures. Results were compared with observations and with large-ensemble simulations performed with an atmospheric general circulation model (MRF9). We focus our analysis on the extratropical stratosphere and its coupling with the troposphere. In this context, the circulation over the North Atlantic sector is particularly important. Relative to the La Niña winter 1989, observations for the El Niño winter 1987 show a negative North Atlantic Oscillation index with corresponding changes in temperature and precipitation patterns, a weak polar vortex, a warm Arctic middle stratosphere, negative and positive total ozone anomalies in the tropics and at middle to high latitudes, respectively, as well as anomalous upward and poleward Eliassen-Palm (EP) flux in the midlatitude lower stratosphere. Most of the tropospheric features are well reproduced in the ensemble means in both models, though the amplitudes are underestimated. In the stratosphere, the SOCOL simulations compare well with observations with respect to zonal wind, temperature, EP flux, meridional mass streamfunction, and ozone, but magnitudes are underestimated in the middle stratosphere. With respect to the mechanisms relating ENSO to stratospheric circulation, the results suggest that both, upward and poleward components of anomalous EP flux are important for obtaining the stratospheric signal and that an increase in strength of the Brewer-Dobson circulation is part of that signal.

ENSO influence on Europe during the last centuries

El Nino/Southern Oscillation (ENSO) affects climate not only in the Pacific region and the tropics, but also in the North Atlantic-European area. Studies based on twentieth-century data have found that El Nino events tend to be accompanied in late winter by a negative North Atlantic Oscillation index, low temperatures in northeastern Europe and a change in precipitation patterns. However, many questions are open, for example, concerning the stationarity of this relation. Here we study the relation between ENSO and European climate during the past 500 years based on statistically reconstructed ENSO indices, early instrumental station series, and reconstructed fields of surface air temperature, sea-level pressure, precipitation, and 500 hPa geopotential height. After removing years following tropical volcanic eruptions (which systematically mask the ENSO signal), we find a consistent and statistically significant ENSO signal in late winter and spring. The responses to El Nino and La Nina are close to symmetric. In agreement with studies using twentieth-century data only, the ENSO signal in precipitation is different in fall than in late winter. Moving correlation analyses confirm a stationary relationship between ENSO and late winter climate in Europe during the past 300 years. However, the ENSO signal is modulated significantly by the North Pacific climate. A multi-field cluster analysis for strong ENSO events during the past 300 years yields a dominant pair of clusters that is symmetric and represents the ‘classical’ ENSO effects on Europe.

ENSO impact on midlatitude circulation patterns in future climate change projections

The remote influence of the leading mode of interannual variability in the Tropics, the El Niño/Southern Oscillation (ENSO), on the northern hemispheric midlatitude circulation in future climate is investigated. For this, IPCC SRES scenarios of the latest version of the coupled climate model ECHAM5/MPI‐OM are used. In ensembles of future climate change projections it is found, that a changing state of ENSO with increased variability has a pronounced influence on the dominant midlatitude circulation pattern, namely the Pacific North America (PNA) pattern and the North Atlantic Oscillation (NAO). More explicitly, in the 21st and 22nd century, a positive (negative) phase of ENSO is more likely followed by a positive (negative) PNA index and negative (positive) NAO index than it is observed in the 20th century. Correlation coefficients between the winter mean Niño3.4 index and the NAO index increase substantially from the 20th century.

Changing relationship between the North Atlantic Oscillation and key North Atlantic climate parameters

The North Atlantic Oscillation (NAO) is the key indicator of long‐term variability in the North Atlantic. Numerous studies have accepted as a paradigm a steady relationship between the NAO and key North Atlantic climate parameters like the sea surface temperature (SST), surface air temperature (SAT), and sea level pressure (SLP). However, some studies suggest that this relationship is not always steady. For example, the recent decline of the Arctic ice cap is accompanied by a neutral or negative NAO index, whereas the ice decline observed over the last decades is associated with the positive NAO phase and with high atmospheric cyclonicity. In this study, we point to a lack of steadiness in the relationship between the NAO and SAT, SST, and SLP over the North Atlantic region when observed over long (decadal) time intervals. This suggests that the relationship is more complex than previously thought and may require further investigation.

Precipitation changes and variability in Turkey linked to the North Atlantic oscillation during the period 1930–2000

AbstractRelationships between the variability of the North Atlantic oscillation (NAO) indices and the normalized precipitation at 78 stations in Turkey, and the influences of the extreme NAO index (NAOI) episodes and the year‐to‐year and longer time‐scale variations in the NAO on the precipitation conditions were investigated. The results of the study have led to the following main conclusions and evaluations for Turkey:(i) There is a negative relationship between interannual variability of the Turkish precipitation series and the NAO indices. Negative relationships that are particularly strong in winter and partly in autumn are detected to be weaker in spring and almost non‐existent in summer. Correlation coefficients are significant at 61 stations in winter, whereas they are significant at 23 and eight stations in autumn and spring respectively. (ii) Composite precipitation means corresponding to the extreme NAOI phases mostly exhibit an apparent opposite anomaly pattern, except in summer, between the negative and positive NAOI phases. (iii) Annual, winter, spring, autumn and partly the summer composite precipitation means are mostly characterized by wetter than the long‐term average conditions during the negative NAOI phase, whereas the positive NAOI responses mostly exhibit drier than the long‐term average conditions annually and in all seasons except summer. (iv) Spatially coherent and statistically significant changes in the precipitation amounts during the extreme NAOI phases are more apparent in the west and mid Turkey. (v) Low‐frequency fluctuations in the circulation over the Atlantic have been closely linked to the coherent large‐scale precipitation anomalies that have persisted, particularly in winter, over Turkey since the early 1960s. (vi) There is a great resemblance between the spatial distribution and magnitude of the negative correlation coefficients and the spatial distribution patterns and severity of the wet (dry) conditions with negative (positive) NAOI phase, annually and in the winter and autumn seasons. (vii) The coherent regions characterized with significant correlation coefficients coincide perfectly with the coherent regions characterized by the extreme NAOI signals. These clear associations increase our confidence with the results. Copyright © 2003 Royal Meteorological Society

Ocean structure and climate (Eastern North Atlantic): in situ measurement and remote sensing (altimeter)

Structure and climate of the east North Atlantic are appraised within a framework of in situ measurement and altimeter remote sensing from 0°–60°N. Long zonal expendable bathythermograph/conductivity–temperature–depth probe sections show repeating internal structure in the North Atlantic Ocean. Drogued buoys and subsurface floats give westward speeds for eddies and wavelike structure. Records from longterm current meter deployments give the periodicity of the repeating structure. Eddy and wave characteristics of period, size or wavelength, westward propagation speed, and mean currents are derived at 20°N, 26°N, 32·5°N, 36°N and 48°N from in situ measurements in the Atlantic Ocean. It is shown that ocean wave and eddy-like features measured in situ correlate with altimeter structure. Interior ocean wave crests or cold dome-like temperature structures are cyclonic and have negative surface altimeter anomalies; mesoscale internal wave troughs or warm structures are anticyclonic and have positive surface height anomalies. Along the Eastern Boundary, flows and temperature climate are examined in terms of sla and North Atlantic Oscillation (NAO) Index. Longterm changes in ocean climate and circulation are derived from sla data. It is shown that longterm changes from 1992 to 2002 in the North Atlantic Current and the Subtropical Gyre transport determined from sla data correlate with winter NAO Index such that maximum flow conditions occurred in 1995 and 2000. Minimum circulation conditions occurred between 1996–1998. Years of extreme negative winter NAO Index result in enhanced poleward flow along the Eastern Boundary and anomalous winter warming along the West European Continental Slope as was measured in 1990, 1996, 1998 and 2001.

Climate variability and glacier reaction in the Austrian eastern Alps

AbstractA carefully homogenized climate dataset is used to interpret glacier behaviour in the Austrian Alps. Periods of glacier advance are generally more maritime and cooler, with reduced sunshine duration and increased precipitation sum during the ablation period. Periods of retreat are associated with a more continental, warmer climate, with increased sunshine duration and reduced precipitation sum. Three recent sub-periods of Austrian glacier behaviour are documented by direct measurement of glacier mass balance (before 1965 more negative; 1965–81 more positive; since 1982 more negative). A long-term mass-balance series in the eastern part of the Austrian Alps parameterized by snow-depth measurements indicates very clearly that periods of more negative mass balance have a higher correlation to summer air temperature and a lower correlation to winter accumulation. Periods of more positive mass balance are highly correlated to winter accumulation and only slightly correlated to summer temperature. The positive mass-balance period 1965–81 is also characterized by negative North Atlantic Oscillation index values which caused an increased meridional circulation mode, resulting in a northwesterly to northerly precipitation regime during winter.

Dependence of winter precipitation over Portugal on NAO and baroclinic wave activity

The relationship between winter (DJF) rainfall over Portugal and the variable large scale circulation is addressed. It is shown that the poles of the sea level pressure (SLP) field variability associated with rainfall variability are shifted about 15° northward with respect to those used in standard definitions of the North Atlantic Oscillation (NAO). It is suggested that the influence of NAO on rainfall dominantly arises from the associated advection of humidity from the Atlantic Ocean. Rainfall is also related to different aspects of baroclinic wave activity, the variability of the latter quantity in turn being largely dependent on the NAO. A negative NAO index (leading to increased westerly surface geostrophic winds into Portugal) is associated with an increased number of deep (ps<980 hPa) surface lows over the central North Atlantic and of intermediate (980<ps<1000 hPa) surface lows over North-western Europe. It is suggested that these distant surface lows have no direct influence on local Portuguese precipitation, but rather contribute to advection at their southern flanks. The other aspect of baroclinic wave activity varying with the NAO is the mid-tropospheric storm track (defined by the 500 hPa bandpass-filtered geopotential height variance). A possible local influence of the storm track due to vertical motions ahead of the upper air troughs cannot be unambiguously separated from the effect of advection. A separate influence of local surface cyclones over the Iberian peninsula which may, for instance, arise from the large scale ascent of air, is revealed by the statistics: for a given advection, rainfall amounts for months with local cyclone cores over the considered region tend to exceed those without. Copyright © 1999 Royal Meteorological Society

Synoptic associations of winter climate and snowfall variability in New England, USA, 1950–1992

Seasonal snowfall in New England has exhibited wide variations over the last few decades, the underlying causes of which have not been identified previously. In this paper, compositing and statistical analyses examine how interactions among the large-scale atmospheric circulation, cyclonic activity and storm track preferences, and western Atlantic sea-surface temperatures contribute to winter climate variability and exceptional winter snowfall totals in New England. High winter snowfall totals are associated with a meridional circulation regime, as indicated by a negative North Atlantic Oscillation (NAO) index and negative 700 hPa height anomalies over the eastern USA. Anomalously high surface pressure over northern Canada promotes advection of cold air into the region, and the preferred storm track is along or just to the south of the New England coast. Low winter snowfall totals are associated with no unique set of circumstances, but a positive NAO index and more zonal circulation is indicated, along with a weaker surface high over Canada, and a storm track over interior New England rather than along the coast. There is no direct association with the Pacific–North American index, although this may modulate the association with the NAO. In extreme southern New England, high (low) winter snowfall totals are associated with negative (positive) sea-surface temperature anomalies along the Atlantic seaboard. © 1998 Royal Meteorological Society

Multiyear variability in the near‐surface temperature structure of the midlatitude western North Atlantic Ocean

Between 1966 and 1995, subsurface temperature data have been collected in the western North Atlantic Ocean using expendable bathythermographs. Data coverage is sparse in both time and space, but evidence for decadal variability in the upper 400 m of the water column is found. The data were averaged by month onto a 2° of latitude by 4° of longitude grid. Thirty‐one quadrangles in the region bounded by 17°N and 43°N and 78°W and 66°W have sufficient data to provide consistent results. Anomaly time series at 0, 100, 200, 300, and 400 m were estimated by subtracting a mean monthly climatology. The individual records were detrended and filtered to highlight the longer‐period signals. The analysis resulted in 25‐year records (1969–1993) for study. Within the thermocline of the subtropical gyre and the Gulf Stream at 100 and 200 m, periods of predominately positive temperature anomaly end in 1971, 1982, and 1990, while periods of negative anomaly end in 1976 and 1985. Only the events ending in 1971, 1976, and 1990 are in the majority of the records at 300 and 400 m. Most of the events also appear in the sea surface temperature (SST) records but are somewhat masked by significant noise at the surface. Meridional‐vertical temperature sections through the subtropical gyre show that transitions from negative to positive anomaly events are characterized by a deepening of the isotherms throughout the section and transitions from positive to negative events by a rising of the isotherms. Significant lateral migration of the axis of the Gulf Stream, although possibly masked by the 2° averaging, is not necessary to explain either type of event. The transitions in the SST and 100‐m temperature time series occur at essentially the same time as the transitions in an index of the North Atlantic Oscillation (NAO) that has also been detrended (i.e., 1971, 1976, 1980, 1984, 1988). The 1971, 1976, and 1988 NAO events are also observed at 300 and 400 m as described earlier. Periods of positive subsurface temperature anomaly are coincidental with periods of positive NAO index, and periods of negative subsurface temperature anomaly are coincidental with periods of negative NAO index. Thus earlier results showing connections between the NAO and western Atlantic SST at decadal timescales are now extended to at least 400 m in the water column. Trends were computed from the individual 25‐year records. The trends at all depths are predominately negative north of 38°N and positive south of 38°N. Inferences from the horizontal distribution of the trends and results from earlier studies suggest that the 1969–1993 period may be a phase of a 30‐ to 50‐year signal observed in the northern Atlantic since the beginning of the century.